1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly, to an LCD driver which has a digital to analog converter (DAC) requiring a plurality of channels.
2. Discussion of the Related Art
A digital to analog converter (DAC) is one of the most important components of a source driver of a thin film transistor (TFT) LCD. A DAC produces an analog output voltage in response to applied digital data. The DAC includes decoder switches selectively outputting one of a plurality of input voltages corresponding to applied data. The DAC has 64 analog switches and 64 decoders in order to select one of input voltages corresponding to the applied data to indicate one of 64 gray levels. Thus, for example, for an 80-pixel color line there are 240 output channels, 64.times.240 analog switches are required, and the 64 input voltages are connected to each analog switch.
A conventional LCD driver will now be described.
FIG. 1 is a block diagram of the conventional LCD driver.
As shown in FIG. 1, the conventional LCD driver includes a shift register 11 for shifting stored R, G, and B data. A first latch 13 stores the R. G and B data sequentially. A second latch 15 holds one line of R, G and B data, stored in the first latch 13 upon application of load signals LP1 through LP80 from the shift register 11. The shift register is 80 bits wide in this example. A bit converter 17 converts the R, G and B data, which is 6-bits, to 13 bit data. A decoder 19 outputs an analog voltage for each channel of the bit converter 17. An output buffer 21 receives analog voltages outputted from the decoder 19 and transmitting them to an LCD panel.
Each of the R, G and B data is 6-bits wide, and there are 240 channels in this example. The first latch 13 stores the R, G and B data in the first channel to the 240th channel in the order of R, G and B. The second latch 15 loads the R, G and B data one line at a time from the first latch 13, and when the load control signal is applied to the second latch 15, the second latch 15 outputs the R, G and B data to the bit converter 17. The bit converter 17 converts 6-bit data to 13-bit data, corresponding to converting voltages in a range of 0 to 5V to voltages in a range of 0 to 12V. Each channel has data corresponding to the voltage in the range of 0 to 12V. Thus, the decoder 19 selectively outputs one of the 128 inputted analog voltages corresponding to each channel.
The operation of the conventional LCD driver will now be described.
As shown in FIG. 1, the shift register 11 determines whether the R, G and B data will be shifted to the left or right, upon application of an input/output control signal. That is, the shift register 11 determines whether it shifts the R, G and B data either in its 1st shift to the 80th shift or in the 80th shift to the 1st shift. The R, G and B data are sequentially stored in the first latch 13 on determination of whether the shift is to the left or right. The first latch 13 includes 240 channels, and one of the R, G and B data is stored sequentially in each channel. The second latch 15 stores the R, G and B data, stored in the first latch 13, wherein the R, G and B data correspond to an 80-pixel color line on the LCD panel.
When the load control signal is applied to the second latch 15, the second latch 15 outputs the R, G and B data to the bit converter 17. The bit converter 17 converts the 6-bit data to the 13-bit data for each channel voltages in the range of 0 to 5V to the voltages in the range of 0 to 12V. Accordingly, the 6-bit data outputted from each channel becomes the 13-bit data to correspond to the voltage range of 0 to 12V.
If the decoder 19 receives the 128 analog voltages from an R-ladder, it outputs a voltage, equivalent to the voltage level of each channel, to an output buffer 21. Thus, the output buffer 21 outputs one of the 128 analog voltages for each channel, transmitting the voltage to the LCD panel.
The conventional LCD driver requires one DAC for each channel, resulting in an increase in an overall size and complexity of the driver.